I lived in ATL and commuted by bike for several months before I moved to Chattanooga, TN. You're right, it is a very car oriented/non-cycle friendly city. The best advice I have is ride aggresivly and make sure that your bike is locked to anything and everything with as many chains and U-Locks you can carry.

Also, the very best mountian biking GA has to offer (in my opinion) is located an hour north at and arround Fort Mountain State Park. Lots of trails up in that area! As for the bike, buy something with two wheels and get on it.